National builds a scanner chip

SANTA CLARA, Calif.  National Semiconductor Corp.'s just released integrated scanner chip promises to lower the price and improve the quality of desktop color scanners. Though the 36-bit scanner-on-a-chip follows devices from Exar Corp., Burr-Brown and others into an increasingly competitive market, National's LM-9830 fulfills the company's long-term promise to use its analog components as building blocks for mixed-signal integration.

National president Brian Halla has been a most prominent advocate of system-level chips and one of the few to speak of "mixed-signal" integration. Though mixed analog-and-digital devices have been seen as a necessary ingredient for future systems-on-a-chip, technologists have begun backing away from the notion of integrating precision analog on the same chip with digital logic.

Panelists at last February's ISSCC seemed to agree that analog functions are either reworked to be implemented in digital CMOS (in which case, they cease to be considered analog) or left off a chip entirely. Rockwell Semiconductor's early 56-k modem line, for example, consisted of separate data-converter and DSP chips in one semiconductor package.

Increasingly, mixed signal has come to mean mixed technology, in which bipolar transistors and, in some cases, DMOS power drivers are put on the same chip with CMOS logic circuits. But because of the additional masking steps required to create bipolar devices, the technology has been considered impractical for the most cost-sensitive consumer applications.

National's LM9830 represents a new rung on the ladder toward mixed-signal integration. Indeed, the company has long identified the scanner-on-chip project as a prime candidate for "forward integration." In presentations over the summer, marketing vice president Mark Levi described an analog integration process that has reduced scanner electronics from the 20 to 40 ICs common before 1996 to six to 10 ICs in 1996 and to just two ICs in 1998.

The new design builds on earlier parts-based on a 6-MHz, 12-bit-pipeline A/D converter-that incorporate digital offset and shading correction along with such analog processing functions as gain offset and correlated double sampling. The LM9830 adds the sensor clock generator, microstepping motor control, data buffering and parallel port, said product manager Fred Hamilton. SRAM and power transistors for the scanner motor driver are really the only additional semiconductors required to implement a complete PC-based scanner, according to Hamilton.

Like the recently introduced Exar XRD9827, the National part supports both charge-coupled devices and contact image sensors. Unlike the Exar part, which relies on external ASICs and DSPs to provide color correction and motor control, the National part accommodates most of those functions on-chip.

A major difference is that National's implementation is priced at slightly less than $10; Exar expects a $2.50 payback for its part.

Hamilton claims that "the image with the LM9830 was clearly superior. When we scanned a 3-inch x 6.5-inch color image at 300 dpi [dots/inch] with a stock scanner from Fry's Electronics, the job took 66 seconds. When we replaced the electronics with National's scanner-on-a-chip, the same job took 16.5 seconds."